During trabeculectomy, or even glaucoma implant surgery, when aqueous humor is drained from the anterior chamber of the eye to the outside tissue, it causes a severe tissue reaction. One tissue reaction is to encapsulate the aqueous humor by means of fibrous tissue growth commonly known as bleb formation. Avoiding forming a bleb, or minimizing the rate of growth of a bleb around an implanted medical device, especially a glaucoma valve, is highly desirable. After the bleb is formed over an implanted glaucoma device, encapsulating it, the intraocular pressure (IOP) in the eye increases because the fluid in the bleb gets locked in. If the bleb is thin enough and there is sufficient blood vessel growth surrounding the fibrous tissue, the aqueous humor is then carried through the blood vessels back into circulation. Even though this phenomenon is helpful for aqueous transport from the bleb into the subcutaneous tissue, there is still resistance to aqueous flow from the eye into circulation. The density, wall thickness, and type of collagen formation of a bleb encasing an implanted glaucoma valve ultimately become the controlling factors of the intraocular pressure in the anterior chamber of the eye. The greater the outside resistance imposed by the bleb, the greater the intraocular pressure (IOP) as implanted medical devices utilize differential pressure.
The second most common cause of blindness in the world is glaucoma, affecting about 53 million around the world. In glaucoma the blood supply to the retina is reduced or almost stopped due to high intraocular pressure in the eye. This results in retinal ganglion death and optic nerve atrophy. The normal pressure in the human eye is between 10 to 21 millimeters of mercury (mm Hg). Once the onset of glaucoma occurs, control of intraocular pressure (IOP) is very important to prevent further vision loss. It is desired to keep the IOP between 10 to 15 mm Hg. Some implant devices are deficient because they have a high rate of hypotony. An IOP below 5 mm of Hg indicates hypotony is occurring. The Molteno glaucoma implant device sold by IOP Ophthalmics of Costa Mesa, Calif., the Barlvealdt glaucoma implant device sold by Abbot Medical Optics of Santa Ana, Calif., and the Express shunt sold by Alcon of Forth Worth, Tex., have a relatively high rate of hypotony. The Ahmed glaucoma valve sold by New World Medical, Inc. of Rancho Cucamonga, Calif., is a self-regulating implantable valve that has successfully reduced the incidence of hypotony. The Ahmed glaucoma valve is discussed in U.S. Pat. No. 5,071,408, illustrating how it is implanted in the sclera and controls the IOP to within the desired range of 10 to 15 mm Hg. In the Ahmed glaucoma valve fluid from the anterior chamber of the eye flows through a valve outlet onto a plate, and then through the bleb tissue and into blood vessels that carry the fluid to the rest of the patient's body. In many patients the drainage of aqueous body fluid onto subcutaneous tissue leads to the formation of a thick, non-percolating bleb enclosing the valve and eventually preventing its operation.
Bleb formation with excessive fibrosis may inhibit or prevent the intended operation of such medical devices. In the case of non-valve glaucoma devices, all need a bleb to maintain proper intraocular pressure (IOP). Usually the pressure in the bleb is less than the IOP in the anterior chamber of the eye. The walls of the bleb allow the aqueous to permeate into the blood stream through blood vessels that grow into the bleb. Most of these devices fail because the bleb walls become too thick and stop the percolation of aqueous out of the system. Consequently, the external resistance becomes too high for the aqueous to drain out of the anterior chamber.
Avoiding or minimizing bleb formation is also important when implanting other medical devices beside ophthalmological devices. For example, implantable self-regulating valves may be used in neurology where cerebrospinal fluid is drained through a shunt device that drains into the peritoneum.